There are substantially two types of contact lenses presently commercially available: hard lenses and soft lenses. The hard lenses are made of glass, poly(methyl methacrylate), crosslinked copolymers of methyl methacrylate with allyl methacrylate, polystyrene, polycarbonates, and the like. The soft lenses are most often comprised of hydrogels, but they may also be hydrophobic----being made of polysiloxane or of polymers and copolymers of butyl acrylate and methacrylate. The hydrophilic lenses are most often prepared from polymers of 2-hydroxyethyl methacrylate crosslinked with less than one percent of ethylene dimethacrylate or 3-oxypentamethylene dimethacrylate (i.e., ethylene glycol and diethylene glycol dimethacrylate). However, they may also be produced using polymers of 2,3,-dihydroxypropyl methacrylate (glycerol methacrylate) crosslinked with a small amount of hydroxypropanediol dimethacrylate or 1,2,3-propanetriol trimethacrylate (glycerol di- or trimethacrylate) or from copolymers of N-vinylpyrrolidone with lower esters of acrylic or methacrylic acid or from other transparent polymers and copolymers.
All these materials have their individual advantages and shortcomings. For example, the hard lenses made from poly(methyl methacrylate) do not conform to the shape of the eye, whereas the soft lenses do conform but do not allow tear circulation. The hard lenses are also impermeable to oxygen, in contrast to the soft lenses which let through oxygen albeit partially but have better permeability for low-molecular-weight metabolites. The lenses from polysiloxane rubber are permeable to gaseous oxygen however they do not allow the metabolites or bactericide lysozymes dissolved in tears to permeate. Upon subsequent study, it was determined that the direct permeation of oxygen through a contact lens is desirable although the amount of oxygen which permeated through the presently available lenses was not sufficient for proper nourishment of cornea. The use of highly swelling hydrogels to circumvent this problem was attempted, as the permeability of oxygen through this material approaches the oxygen permeability of pure water. However, it was discovered that the majority of oxygen is supplied to the cornea not by means of oxygen permeating through this material, but by tear liquid, the access of which is obstructed by soft hydrogel lenses due to their close fit on the surface of the cornea. Moreover, the more the lens is swollen with water, the lower is its refractive index, and consequently, the thicker it must be. This swelling which results in the favorable characteristic of high oxygen permeability unfortunately also has a negative aspect--it extends the diffusion path of the oxygen. The limited access of tears and thus also that of lysozymes is a very serious defect which allows the undisturbed growth of harmful microbes. It is for this reason that the long-term wearing of very soft and thus well physically tolerated contact lenses is not only undesirable, but very dangerous from a medical perspective. These lenses significantly increase the risk of ulcers on the cornea and thereby endangering a person's sight.
State-of-the-art contact lenses are presently constructed of synthetic hydrogels, in particular lightly crosslinked 2-hydroxyethyl methacrylate. These hydrogels have the least disadvantages but contain up to 40% water when completely swollen. In an attempt to increase the water content of the lenses, manufacturers are presently striving to increase the water content of the lenses to approach that which is contained in the body (about 70%). This, however, requires strongly swelling gels which must exhibit sufficient structural strength to maintain the precise shape of the lens and to resist tearing, e.g., from a damaged edge. Such properties are mutually exclusive in the aforementioned presently available materials. Moreover, the increased water content results in a decrease of the refractive index and necessarily in an increase in thickness, above all in aphakic lenses. Although lysozymes cannot penetrate through a strongly swollen gel because of their high molecular weight, such a gel has the drawback of making the access of tears to the cornea more difficult because the softer the gel is the more it adheres to the eye surface.
In view of the foregoing discussion, it is clear that various portions of a contact lens require so many different characteristics in order to ensure the lens' proper functioning that a single ideal material which would fully meet all of the requirements would seem to be non-existent. The optimal solution to the diverse demands of a contact lens which would allow long-term wearing consists of the application of at least two different materials. As the first hard contact lenses were made of glass or organic glass, it is not surprising that proposals have already been made to provide a glass or other type of hard lens with a softer edge or a continuous base, e.g., regenerated cellulose, rubber, and the like, in order to avoid wounding the eye with the thin glass edge lens and to reduce the irritation of the eye. See, for example, German (FRG) Pat. No. 921,416. However, this idea was not practiced as both materials are so different that they could not be permanently joined. Moreover, the design of lens was faulty, i.e. the lens rested tightly on the surface of cornea and thus prevented the access of oxygen both by diffusion and by the free streaming of tears onto the eye surface. It also prevented the access of lysozymes as the thin layer of swollen regenerated cellulose acted as an efficient seal. Moreover, regenerated cellulose in the swollen state is not softer than the surface of the cornea and it irritated the eye; this fact being mentioned by the inventor of the aforementioned patent.
Another attempt to remove the previously mentioned shortcomings was to provide a row of openings in the edge of a contact lens or in its center so that blinking causes the forced circulation of tear liquid due to the alternate pushing and drawing away of the lens (in particular, the soft lens) from the eye surface. However, the expected effect did not occur to a sufficient extent as the thin lens was too easily stuck to the eye and was therefore not able to effectively pump the tears in the space between the lens and cornea.
The present invention overcomes the shortcomings of the prior art lens with a unique lens design.
The features and advantages of the invention will be apparent upon reading the following description of preferred exemplified embodiments of the invention and upon reference to the accompanying drawings wherein: